Magnetic coating thickness comparator having parallel magnetic rods with nonmagnetic slide indicators

ABSTRACT

A coating thickness and surface comparator for measuring the thickness of nonmagnetic coatings (28) or roughness on ferromagnetic base materials. A permanent rod magnet (6) and a ferromagnetic rod (16) are supported by nonmagnetic slides (12, 14) parallel to one another. When a standard base (24) is placed at one end (22) of the rod magnet (6) and a coated or roughened base (30) is placed at the other end, the slides (12, 14) are pushed together toward the middle of the rod magnet (6). The coincidence of the slides (12, 14) varies from the middle of the bar magnet (6) in proportion to coating thickness (28) or roughness.

This is a continuation-in-part of application Ser. No. 320,508, filedNov. 12, 1981, now abandoned.

TECHNICAL FIELD

The field of the invention is electricity, measuring and testing whereina layer or layered material thickness is measured using a magnetic fieldto test the material. The present invention is particularly concernedwith the measurement of the thickness of nonmagnetic coatings or layerson magnetic base materials.

BACKGROUND ART

The state of the art of measuring the thickness of nonmagnetic coatingson a magnetic base using a magnetic field may be ascertained byreference to U.S. Pat. Nos. 2,469,476; 2,625,585; 2,903,645; 3,521,160and 3,699,487, the disclosures of which are incorporated herein byreference.

U.S. Pat. No. 2,469,476 of Sellers discloses an instrument for measuringthe thickness of nonmagnetic coatings on a ferrous base and the patentalso discloses that it is possible to use the instrument for measuringthe irregularities in the surface of a machined ferrous material.According to this patent an adjustable bar magnet bridge is pivotedbetween the legs of a U-shaped magnet. The limbs of the U-shaped magnetare placed on the surface to be measured. The pivoted bar magnet has anindicator attached thereto and is spring biased so that changes in themagnetic flux of the U-shaped magnet indicate the layer thickness orroughness.

The magnetic measuring gauge disclosed in U.S. Pat. No. 2,625,585 makesuse of the principle that forces necessary to disengage a magnet from aworkpiece vary in inverse proportion to the thickness of the nonmagneticplating or coating disposed upon the magnetic base surface. U.S. Pat.No. 2,903,645 makes use of this same principle but in a more complexinstrument.

U.S. Pat. No. 3,521,160 is representative of a series of patents of Nixet al which show magnetic thickness gauges using a magnet mounted on theend of a balanced rotatable arm to which a dial is coupled over aspring. The dial is turned until the increasing tension of the springbreaks the magnetic contact and the thickness is read directly from thedial.

Steingroever discloses in U.S. Pat. No. 3,699,487 magnets of anisotropicmaterials for use in the magnetic thickness gauges.

According to these magnetic thickness gauges of the prior art, themagnet is placed on a coated steel surface and pulled off by a spring.The force required to pull the spring off of the surface is indicated ona scale and expressed in coating thickness. The spring force and themagnetic force are of an uncomplimentary or non-linear nature andproduce a logarithmic scale. As time goes on, the magnet either loses orgains magnetism and the spring is subject to fatigue and temperaturevariations. To obtain a fair degree of accuracy, each prior art gaugehas to be individually calibrated and requires an elaborate set-up whichis very time consuming. These so-called "pull-off gauges" are calibratedto a mild steel plate. As soon as the coated steel surface is of a roughnature or the substrate is of a different steel than that to which thegauge is calibrated, the readings of these gauges become relative.

OBJECTS AND BRIEF DISCLOSURE OF INVENTION

Having in mind the limitations of the prior art, it is an object of thepresent invention to provide a coating thickness and surface roughnesscomparator which avoids the uncomplimentary or non-linear reactionbetween magnetic force and spring force.

This object is accomplished in the present invention by using theprinciple of magnetic force versus friction.

In a particular embodiment of the comparator of the present invention,the comparator has the following elements:

(a) a permanent rod magnet or a rod having equal magnets at each end;

(b) a ferromagnetic rod approximately parallel to the permanent rodmagnet or the rod having equal magnets at each end;

(c) nonmagnetic slides or spacers preventing contact between the rodmagnet and the ferromagnetic rod; and

(d) an indicator of the middle of the rod magnet.

The comparator is used for making measurements by placing a first orreference pole of the rod magnet in contact with a referenceferromagnetic base material. The second or measuring pole of the rodmagnet is placed in contact with the coated or roughened ferromagneticsurface to be measured. Both slides or spacers are moved to theextremities of the measuring and reference poles, respectively, and thenmoved together by hand or other means. An indication of the center ormiddle position of the rod magnet is provided and deviations from thiscenter point by the coincidence of the slides is an indication of thethickness and/or roughness of the surface being measured.

DESCRIPTION OF THE DRAWINGS

The invention is further explained by reference to the accompanyingdrawings showing the preferred embodiment of the present invention,wherein:

FIG. 1 is a side view in partial cross-section of the coating thicknessand surface roughness comparator of the present invention as used with areference surface and a coated surface;

FIG. 2 is a bottom plan view of FIG. 1; and

FIG. 3 is a cross-sectional view of FIG. 1 along the line III--III.

DETAILED DESCRIPTION OF THE BEST MODE FOR CARRYING OUT THE INVENTION ANDOF THE PREFERRED EMBODIMENT

With particular reference to FIG. 1, the comparator is shown having ahousing or casing 2 with a scale 4 thereon. The scale 4 is calibratedwith a plus scale on one side of the center or middle of the permanentrod magnet 6 and a minus scale on the other side of the middle of therod magnet.

The rod magnet 6 is supported in holes 8,10 at the ends of the housing.The housing 2 is composed of a nonmagnetic material and the holes 8,10can hold the rod magnet 6 securely by friction so that the middle of therod magnet is in register with the zero indication of the scale 4.

Nonmagnetic slides or spacers 12, 14 support a ferromagnetic rod 16substantially parallel to the rod 6. The ferromagnetic rod 16 ispreferably a cold rolled steel rod. The ends 15 of the housing 2 preventa horizontal movement of the ferromagnetic rod 16.

An adjustable spacer 18 is releasably fixed to the housing 2 by a screw20. An uncoated steel plate 24 is positioned adjacent a reference pole22 (one end of the rod 6) and a measuring pole 26 (the other end of therod 6) is placed against a coating 28 on a steel plate 30. The spacer 18is provided with a V-notch on one end and is adjustable so as toaccommodate a rod (not shown) rather than the plate 24 if desired.During operation of the device (either calibration or normal use) theuncoated steel plate 24 (or a rod) is releasably fixed at the end of thedevice by an elastic member or spring 17. The spring 17 passes over twoarms 39 which extend from either side of the housing 2. The arms 39 areprovided with grooves so as to securely receive the ends of the spring17. In this way, the plate 24 is releasably urged against the end of therod magnet 6 and the end of the adjustable spacer 18.

The nonmagnetic materials: the housing 2, the spacers 12 and 14, theadjustable spacer 18 and the screw 20, are suitably made ofunplasticized plastics such as Nylon 6--6 or copper base metals such asbrass.

In FIG. 2, the adjustable spacer 18 is shown with an adjustment slot 19so as to permit the spacer 18 to be moved relative to the housing whenthe plate 24 is replaced by a rod (not shown).

The slider 14 is shown in FIG. 3 with two notches 32, 34 engaging alongitudinal opening 36 of the housing 2 and also supporting the rods 16and 6 in the holes 38 and 40, respectively.

A method of using the present invention is discussed with reference toFIG. 1. To measure the thickness of the nonmagnetic coating 28 on thebase 30, the measuring pole 26 is placed adjacent to the coating. Themeasuring pole 26 is preferably arranged perpendicularly relative to thecoating to be measured. Likewise, the reference pole 22 is preferablyarranged perpendicular to the standard plate 24. Both the base 30 andthe standard plate 24 are preferably of the same ferromagnetic material,namely steel, and preferably are of the same thickness or roughness.

If desired, the base 30 (and the coating to be measured) may also bearranged either horizontally or vertically with the measuring pole 26arranged at an angle of 45° relative to the base 30. In this way, anyeffect of gravity on the accuracy of the device is minimized bymaintaining the device at an angle of 45° to the horizon.

If the device is to be oriented at an angle of 45° with respect to thecoating to be measured, the rod 6 preferably extends sufficiently beyondthe ends of the housing so that the device may be readily oriented atthe angle of 45° with the end of the rod 6 in contact with the coatingto be measured.

Whether the device is horizontal, vertical or at an angle of 45°, thesliders 12 and 14 are each moved outwardly toward the measuring pole 26and the reference pole 22, respectively, and the sliders are then pushedtogether (i.e., toward one another) until they coincide.

Due to the coating 28, the measuring pole is not attracted to the base30 to the same extent as the reference pole 22 is attracted to thestandard plate 24. This magnetic attraction of the pole 26 to the base30 varies inversely according to the thickness of the coating 28 and asa result of the reduced magnetic attraction, the rod 16 is attractedmore strongly at the measuring pole 26.

As a result of this increased magnetic attraction between the measuringpole 26 and the rod 16, more force is applied to the slider 12 so thatwhen the slider 12 is moved to the right to coincide with the silder 14,increased friction is applied. As a result of the increased frictionbetween the slider 12 and the rods 6 and 16, the slider 12 drags andwill be positioned to the left of the zero reading of the scale 4 whenthe two sliders 12, 14 meet. The reading on the scale 4 is an indicationof the thickness of the coating 28.

The device according to the present invention may also be operated inother manners. For example, the standard plate 24 may be replaced by astandard plate having a known coating thickness and as a result thickercoatings can be measured at the measuring pole 26 without exceeding therange of the scale 4.

In yet another method of operation, the reference pole 22 is firstplaced preferably at an angle of 45° to the horizon and contacts astandard plate (not shown) which is arranged either horizontally orvertically. There is no plate provided at the measuring pole 26.

Again the sliders 12 and 14 are moved to the measuring pole 26 and tothe reference pole 22, respectively, and they are then pushed togetheruntil they coincide and a first reading to the left of zero on scale 4is recorded.

Now the reference pole 22 is placed adjacent to and preferably at anangle of 45° with respect to the base 30 having a nonmagnetic coating 28and again there is no plate at the measuring pole 26.

The sliders 12 and 14 are moved to the measuring pole 26 and to thereference pole 22, respectively, and they are then pushed together untilthey coincide and a second reading to the left of zero on the scale 4 isrecorded.

The difference between the first reading recorded and the second readingis representative of the thickness of the coating 28.

The principles, preferred embodiments and modes of operation of thepresent invention have been described in the foregoing specification.The invention which is intended to be protected herein should not beconstrued as limited to the particular forms disclosed, since these areto be regarded as illustrative rather than restrictive. Variations andchanges may be made by those skilled in the art without departing fromthe spirit of the present invention.

What is claimed is:
 1. A coating thickness comparator comprising:(a) apermanent rod magnet having a middle, a reference pole and a measuringpole; (b) a ferromagnetic rod arranged parallel to said permanent rodmagnet; (c) first and second nonmagnetic slides supporting saidpermanent rod magnet and said ferromagnetic rod and maintaining saidrods substantially parallel to one another said slides being arrangedfor longitudinal movement along said rods; (d) a housing including meansfor indicating said middle of said permanent rod magnet; and (e) astandard ferromagnetic base abutting said reference pole, and aferromagnetic base having a coating thereon abutting said measuringpole; a position of said nonmagnetic slides when pushed together tocoincide indicating a thickness of said coating.
 2. The thicknesscomparator of claim 1, wherein said housing is nonmagnetic and saidslides are suspended in the housing and said indicating means includes ascale on said housing parallel to said rods.
 3. The thickness comparatorof claim 2, further comprising an adjustable spacer on said housingreleasably mounted adjacent said reference pole.
 4. The thicknesscomparator of claim 3, wherein said housing, slides, and adjustablespacer are made of plastic.
 5. The thickness comparator of claim 1,wherein both of said poles extend beyond the ends of said housing. 6.The thickness comparator of claim 1 further comprising a reference platereleasably mounted adjacent said reference pole.
 7. The thicknesscomparator of claim 1 wherein said one of said poles extendssufficiently beyond said one end of said housing whereby said thicknesscomparator may be oriented at 45° to said coating thickness to bemeasured.
 8. A coating thickness comparator for measuring a coatingthickness, comprising:a first rod having magnetic poles at either end; asecond rod arranged substantially parallel to said first rod, saidsecond rod being magnetically attracted to said poles of said first rod;means for supporting said second rod with respect to said first rod,said means for supporting including first and second unmagnetizableslides arranged for longitudinal movement along said first and secondrods; a housing including means for indicating a mid-point of said firstrod, at least one of said poles of said first rod extending beyond saidhousing, said housing including indicating means for measuring adistance of said first and second slides relative to the mid-point ofsaid first rod.
 9. The coating thickness comparator of claim 8 whereineach of said slides includes first and second openings, said first rodbeing slidably received within the first opening of each of said slidesand said second rod being slidably received within the second opening ofeach of said slides, a frictional force between one of said slides andthe first and second rods corresponding to the coating thickness to bemeasured when said coating thickness is adjacent to one of the magneticpoles of the first rod.
 10. The coating thickness comparator of claim 9wherein said housing is unmagnetizable and wherein said indicating meansincludes a scale provided on said housing parallel to said first andsecond rods.
 11. The coating thickness comparator of claim 10 wherein amid-point of said second rod is aligned with a mid-point of said scale.12. The coating thickness comparator of claim 8 wherein said first rodcomprises a permanent rod magnet and said second rod comprises a rod offerromagnetic material.
 13. The coating thickness comparator of claim 12wherein a mid-point of said second rod is aligned with a mid-point ofthe first rod.
 14. The coating thickness comparator of claim 8 furthercomprising an adjustable spacer on said housing releasably mountedadjacent said reference pole of the first rod.
 15. The coating thicknesscomparator of claim 14 wherein the housing is of plastic material andwherein the slides and the adjustable spacer are of plastic material.16. The coating thickness comparator of claim 8 wherein both poles ofsaid first rod extend beyond said housing.
 17. The coating thicknesscomparator of claim 9 wherein the poles of said first rod define areference pole and a measuring pole, each of said poles extending beyondsaid housing means,a frictional force between both of said slides andsaid first and second rods corresponding to the coating thickness to bemeasured when both a standard ferromagnetic base is adjacent thereference poles and the coating thickness to be measured is adjacent themeasuring pole.
 18. The coating thickness comparator of claim 17 whereinsaid indicating means includes a scale provided on said housing parallelto said first and second rods and wherein said slides are arranged formovement both away from one another to first and second ends of thescale and toward one another.
 19. A method for measuring a coatingthickness comprising the steps of:contacting a surface of a coating tobe measured with a first magnetic pole of a first rod; maintaining asecond rod substantially parallel to said first rod with first andsecond spacers arranged for sliding movement along said first and secondrods; magnetically attracting said second rod toward said first rod, theattraction of a first end of said second rod to said first magnetic poleof the first rod corresponding to the thickness of said coating to bemeasured; sliding said first and second spacers toward one anotherwhereby the attraction between said first and second rods produces afriction between the first spacer and said rods corresponding to thethickness of said coating to be measured; measuring a position of saidfirst and second spacers when adjacent one another relative to amid-point of the first rod to indicate the thickness of the coating tobe measured.
 20. The method of claim 19 further comprising the stepsof:simultaneously contacting a surface of a standard ferromagnetic basewith a second magnetic pole of the first rod, said simultaneouscontacting producing a friction between the second spacer and said rodscorresponding to the attraction between the second magnetic pole and thestandard ferromagnetic base.
 21. The method of claim 19 furthercomprising the steps of:contacting a surface of a standard ferromagneticbase with the first magnetic pole of the first rod; magneticallyattracting said second rod toward said first rod; sliding said first andsecond spacers toward one another until the first and second spacerscoincide; measuring a position of said first and second spacers relativeto a mid-point of the first rod; sliding said first and second spacersaway from one another toward the poles of the first rod; separating thesurface of the standard ferromagnetic base from the first magnetic poleof the first rod; comparing the measured position of the first andsecond spacers when the first pole contacts the surface of the standardferromagnetic base with the measured position of the first and secondspacers when the first pole contacts the coating to be measured.